1. Field of the Invention
The present invention relates to an offset apparatus for an NC machine tool, and more particularly to an offset apparatus for offsetting the amount of operation of a motion mechanism in accordance with the result of performance analysis of the NC machine tool.
2. Description of the Prior Art
Operation of an NC machine tool is controlled by a numerical controller provided for the NC machine tool. More specifically, the numerical controller sequentially analyzes an NC program and generates operation command signals for spindle motor, servo motor of feed system, etc. and, based on the thus generated operation command signals, the spindle motor, feed system servo motor, etc. are controlled for operation.
Here, the condition of the NC machine tool is not constant at all times but varies from moment to moment according to its operation condition. For example, the cutting edge of the tool wears due to machining, and the machining accuracy gradually degrades. Therefore, to ensure proper machining accuracy, it is commonly practiced to offset the amount of operation of the feed system servo motor by the amount of wear.
Further, the bearing for supporting the spindle generates heat during its rotation due to the friction between the rolling element and bearing ring, and the generated frictional heat is transmitted to the spindle, causing the temperature of the spindle to rise and deforming the spindle due to the thermal expansion caused by the temperature rise. As a result, the relative positional relationship between the tool and work changes, resulting in a machining error. In view of this, it is commonly practiced to measure the spindle temperature by a temperature sensor, estimate the amount of thermal deformation of the spindle from the measured spindle temperature and, based on the estimated amount of thermal deformation, offset the amount of operation of the feed system servo motor so as to compensate for the thermal deformation.
Likewise, the bearing for supporting the feed screw in the feed system also generates heat during its rotation due to the friction between the rolling element and bearing ring, and the generated frictional heat is transmitted to the feed screw, causing the temperature of the feed screw to rise and deforming the feed screw due to the thermal expansion caused by the temperature rise. As a result, the positioning accuracy of the feed system degrades, resulting in a machining error. In view of this, it is commonly practiced to detect the thermal deformation of the feed screw by a displacement sensor and, based on the detected amount of thermal deformation, offset the amount of operation of the feed system servo motor so as to compensate for the thermal deformation.
However, the heat generated at the above bearings is transmitted not only to the spindle and the feed screw but, via these parts, to the entire structure such as the bed, headstock, spindle head, saddle, and column that constitute the machine tool, and thus the entire structure suffers thermal deformation, causing the relative positional relationship between the tool and work to change and resulting in a machining error. Therefore, to achieve high accuracy machining, not only must offsets be applied for the deformations of the spindle and the feed screw caused by the temperature rise of the bearings, but the thermal deformation of the entire structure must also be analyzed so that offset appropriate to the thermal deformation can be applied. In the prior art, however, such advanced offset has not been performed.
Further, the machine tool generates vibrations during machining, and if the frequency is close to the natural frequency of the machine tool, the machine tool resonates, and the effects of the vibrations are transferred to the machined surfaces, degrading the machining accuracy and the quality of the machined surfaces. Accordingly, to achieve high accuracy, high quality machining, it is preferable that both the frequency during machining and the natural frequency of the machine tool be analyzed and the rotational speed and/or feed speed of the tool and work be offset so as to prevent the above frequencies from coming close to each other.
The present invention has been devised in view of the above situation, and it is an object of the invention to provide an offset apparatus for an NC machine tool, that can achieve high accuracy, high quality machining by analyzing the behavior of the NC machine tool (the performance of the machine tool motion mechanism) and by offsetting the amount of operation of the motion mechanism in accordance with the result of the analysis.
The present invention which achieves the above object concerns an NC machine tool offset apparatus that is provided for an NC machine tool equipped with a numerical controller for controlling operation of a motion mechanism in accordance with an operation command signal, and that offsets the amount of operation of the motion mechanism which is driven and controlled in accordance with the operation command signal, comprising:
analysis data storing means for storing three dimensional model data of the motion mechanism and condition data for performance analysis;
analyzing means for analyzing the performance of the motion mechanism, based on the operation command signal in the numerical controller and on the three dimensional model data of the motion mechanism and the condition data for performance analysis stored in the analysis data storing means;
data accumulating means for storing performance analysis data analyzed by the analyzing means;
offset amount computing means for computing, from the performance analysis data stored in the data accumulating means, the amount of offset to be applied to a commanded operation amount directed by the operation command signal; and
offset executing means for offsetting the amount of operation of the motion mechanism, based on the amount of offset computed by the offset amount computing means.
The motion mechanism in the present invention collectively refers to the mechanisms constituting the machine tool excluding the controller, and includes: structures such as a bed, table, spindle, headstock or spindle head, saddle, and column; a feed mechanism comprising a feed screw, nut, feed motor, etc.; a spindle motor; and peripheral devices such as a tool changer and a pallet changer.
According to this offset apparatus, first the analyzing means analyzes the performance of the motion mechanism, based on the operation command signal in the numerical controller and on the three dimensional model data of the motion mechanism and the condition data for performance analysis stored in the analysis data storing means, and the analyzed performance analysis data is stored in the data accumulating means. Next, the offset amount computing means computes, from the performance analysis data stored in the data accumulating means, the amount of offset to be applied to the commanded operation amount directed by the operation command signal; then, based on the amount of offset computed by the offset amount computing means, the offset executing means offsets the amount of operation of the motion mechanism. Here, a technique such as a finite element method or boundary element method is used as the technique for analysis.
Examples of the performance of the motion mechanism include, besides the deformation of the motion mechanism due to load, the natural frequency of the motion mechanism itself and vibrations caused by machining, but the performance is not limited to these factors. Examples of the load include, besides the thermal load from a heat generating source such as a bearing and the machining load due to machining, a varying load the acting point of whose own weight varies due to the movement of a movable body such as a saddle, table, or column.
In this way, when offsetting for the deformation of the motion mechanism due to thermal load, for example, first the amount of heat generated by heat generating sources, such as the spindle supporting bearing and the feed screw supporting bearing, is computed by the analyzing means, based on the operation command signals for the spindle motor and the feed motor (signals relating to rotational speed) received from the numerical controller; then, based on the amount of generated heat thus computed and the data stored in the analysis data storing means, the amount of deformation of the entire motion mechanism is computed by the analyzing means by using the above analysis technique. Next, based on the amount of deformation thus computed, the amount of displacement, for example, in the relative positional relationship between the work and tool is computed by the offset amount computing means, and the amount of operation offset that compensates for the amount of displacement is computed for each feed mechanism. Then, the offset is executed by the offset executing means in accordance with the thus computed amount of offset.
Further, when offsetting for the deformation of the motion mechanism due to machining load, first the machining load is computed by the analyzing means, based on the operation command signals, etc. for the spindle motor and the feed motor (signals relating to spindle rotational speed, feed speed, or electric current value of each motor) received from the numerical controller; then, based on the machining load thus computed and the data stored in the analysis data storing means, the amount of deformation of the entire motion mechanism is computed by the analyzing means by using the above analysis technique. Next, based on the amount of deformation thus computed, the amount of deformation in the relative positional relationship between the work and tool is computed by the offset amount computing means, and the amount of operation offset that compensates for the amount of deformation is computed for each feed mechanism by the offset amount computing means. Then, the offset is executed by the offset executing means in accordance with the thus computed amount of offset.
On the other hand, when making an offset by analyzing the vibrations of the motion mechanism, first the analyzing means, based on the data stored in the analysis data storing means, analyzes the natural frequency of the motion mechanism by using the above analysis technique and, at the same time, analyzes the vibrations being caused by machining, based on the operation command signals for the spindle motor and the feed motor (signals relating to spindle rotational speed, feed speed, or electric current value of each motor) received from the numerical controller and on the data stored in the analysis data storing means. Then, the offset amount computing means compares the frequency due to machining with the natural frequency thus analyzed, and if the frequency due to machining is within a predetermined range relative to the natural frequency, the amount of offset (the amount of decrease or increase in speed) to be applied to the feed speed of the feed mechanism and/or the rotational speed of the spindle is computed. Then, the offset is executed by the offset executing means in accordance with the thus computed amount of offset.
In this way, according to the above offset apparatus, since the deformation of the entire motion mechanism caused by thermal load, machining load, etc. is analyzed, and the amount of operation of the feed mechanism is offset based on the amount of deformation thus analyzed, accurate offset can be achieved compared with the prior art. Such accurate offset contributes to further enhancing the accuracy of machining.
Furthermore, since the motion mechanism can be prevented from resonating due to machining vibrations by offsetting the feed speed of the feed mechanism and/or the rotational speed of the spindle, chattering vibrations can be prevented from occurring in the machining process, and the surface accuracy of machined surfaces can thus be enhanced.